The
Gasoline Storage Tanks

Thegasolinesold
at service stations is stored underground in buried tanks. Each holds several
thousand gallons of gas. There are at least two of these tanks per station and
each tank usually holds a different grade of gas. Having the gas tanks
underground presents an obvious problem: If the gas must get to a dispenser (and
your car's gas tank) located above ground, it has to defy gravity in order to
get there -- like a waterfall flowing uphill. But moving the gas from its
subterranean hideaway up to street level isn't as difficult as you might think.

Most
service stations do the job using one of two types of pump -- a submersible pump
or a suction pump:

Asubmersible
pump, as its name implies, is submerged below the surface of the
liquid, where it uses a propellerlike device called animpellerto
move the fuel upward. Slanted blades on the rotating impeller push the water
the way the blades on anelectricfan
push air.

Asuction
pumpmoves the gas
using the principle ofunequal
pressure. A pipe is inserted in the water. A motor above the fluid
level removes enough air from the pipe to decrease the air pressure above
the gasoline. The motor continues to remove air until the air pressure above
the gasoline is lower than the air pressure pushing down on the gas outside
the pipe. The weight of the surrounding air forces the gas inside the pipe
upward even asgravitytries
to pull it back down. When the air pressure inside the pipe is low enough,
the gas simply climbs up into the aboveground dispenser.

The
major advantage of a submersible pump over a suction pump is that the impeller
can push water over longer vertical distances. However, because the gas tanks at
most service stations are located only a few feet below the dispenser, a suction
pump is usually more than adequate for the task at hand. There's more to this
process, though, and we'll explore it further on the next page.

The
Check Valve

The
route that thegastakes
from the tanks to the aboveground dispenser isn't terribly complicated, though
it may take a few minor twists and turns. When pumping is complete and the pump
motor is turned off, the gas inside the pipe doesn't simply fall back into the
tank. Instead, it's held inside the pipe by acheck
valve. The check valve, which is located above the gas inside the pipe,
creates an airtight seal above the fluid. Although the bottom of the pipe
remains open, the vacuum pressure created by the check valve holds the gas in
place. This is a process known askeeping
the prime.

Using
a check valve to hold the gas inside the pipe prevents unnecessary wear and tear
on the suction pump and assures that a supply of gas will remain in the pipe so
that the next customer won't have to wait for it to be drawn all the way up from
the tank. It may not seem like a big deal, but the process can take 10 to 15
seconds. That isn't a very long wait by any means, but it can be an eternity
when you're waiting for gas to be pumped.

The
power that drives the pumps usually comes from the same electric grid that
powers the lights and appliances in your home, though a few states require that
service stations maintain a backup power supply in case of power failure.

Now
that the gas is on the way to thecarand
it's time for the customer to start pumping, how does the dispenser know just
how much gas the customer has pumped? Considering the volatility of gas prices
these days, that may be the only thing the customer may care about. Find out the
key to this mystery on the next page.

The
Flow Meter

As
a driver, your primary objective at the pump is to get your tank filled so that
you can get yourcarback
on the road. The goal of the service station owner and the company that supplies
thegas,
however, is to know just how much gas you've pumped so they can properly charge
you for it. That's where the flow meter comes in.

As
the gasoline travels upward into the dispenser, it passes through aflow
control valvethat
regulates the gasoline's flow speed. It does this via a plastic diaphragm that
gets squeezed more and more tightly into the pipe as the flow of gas increases,
always leaving just enough room for the proper amount of gasoline to get
through. If you've set a predetermined amount of gas to be pumped, the flow of
gas will slow down as you approach the limit.

This
pipe also contains theflow
meter, which is a castironoraluminumchamber
containing a series of gears or a simple rotor that ticks off units of gas as
they pass through. Information about the gas flow is passed on to a computer
located in the dispenser, which displays the metered amount of gas in tenths of
a gallon. As the temperature of the gas changes -- on particularly hot and cold
days, for instance -- the density of the gas may change, causing an error in the
amount of fluid measured by the flow meter. The computer compensates this error
by taking the gas temperature into account as it records the flow and adjusts
the price accordingly.

Wear
and tear on the meter may degrade its accuracy over time, which is why periodic
inspections are necessary. Typically, inspectors will use a container of a
certain volume, pump gas into it and compare the amount in the container with
the amount metered on the dispenser. If the amounts don't match, the flow meter
will need to be recalibrated and possibly refurbished or replaced. Although
regulations for pump calibration come from the National Institute for Standards
and Technology (NIST), the actual inspections are performed locally, usually by
a state's Department of Weights and Measures.

Now
that the gas is flowing and the amount of flow has been measured, there's only
one step left: getting the gas into the customer's car. But that's a trickier
process that you might think. For instance, what if the customer doesn't know
when to stop pumping? Will he or she get soaked in a potentially lethal eruption
of runaway fuel? Let's find out on the next page.

The
Blend Valve

One
of the first things that a customer will notice at the pump is the variety of
choices offered. In most cases, a dispenser will offer several grades ofgas--
sometimes as many as five -- each with a different octane rating. The desired
octane rating is usually chosen simply by pushing a button. Does this mean that
there are five different underground tanks feeding into that dispenser, each
offering a different grade of gas? That's not usually the case. In fact, the
dispenser can produce as many grades as it wants from as few as two underground
tanks, as long as one tank contains the highest grade of octane available at
that station and the other contains the lowest. The grades are blended together
at the pump -- not unlike the way you'd blend gin and vermouth to make a martini
-- producing a kind ofoctanecocktail.
The precise proportion in which the grades are blended determines the octane of
the gas that enters the customer's tank.

This
feat of gas pump bartending is performed by something called ablend
valve. This valve has inputs consisting of two grades of gasoline, each
from different tanks. A single, moveable barrier called ashoeis
connected to both in such a way that it can be moved across the inputs with a
single motor-driven ratchet. As the ratchet opens one valve, it closes the other
valve in precise but opposite proportion. This means that when one valve is, for
example, 90 percent open, the other valve is 10 percent open, creating a mixture
that consists of 90 percent of one octane and 10 percent of the other. By
shifting the ratchet back and forth, the blend valve can produce any octane of
gas, ranging from the highest to the lowest grades stored in the tanks -- and
all octanes in between.

Keep
reading to find out how the dispenser makes sure that you don't overflow the
gasoline capacity of your tank.

The
Automatic Shut-off

When
the customer removes the pump handle from its place on the side of the
dispenser, this action activates a switch that starts the dispenser operation.
(In some cases the switch is spring-loaded and activates automatically; in
others, the customer must raise a small lever manually to begin the process.) At
that point, the customer simply inserts the nozzle into the car's gas tank and
pulls the lever. Stopping the flow ofgasis
just as simple -- the customer need only release the lever to cut off the stream
of fuel.

But
what if the tank fills unexpectedly to the brim and the gasoline threatens to
overflow? As anyone who's ever operated a gas pump knows, the pump will switch
off automatically. But how does the pump know when to stop pumping?

As
the gas level in the tank rises, the distance between the dispenser nozzle and
the fuel grows smaller. A small pipe called aventuriruns
alongside the gas nozzle. When the end of the venturi pipe becomes submerged in
the rising gas, it chokes off the air pressure that holds the nozzle handle open
and shuts down the flow of gas. Unfortunately, this shutdown can sometimes
happen before the tank is full as the rapidly flowing gas backs up on its way
into the tank. This can cause the gas handle to spring open before pumping is
complete, leaving the annoyed customer to squeeze the handle again and risk the
possibility of overflow. Pausing briefly will allow the gas to continue into the
tank and the pump nozzle to start pouring gas again.

For
more information on fuel and fuel efficiency, take a look at the links on the
next page.

How
does a gas pump know when my tank is full?

This
mechanism has been around for a long time, so it is safe to say there is not a
miniaturecamerainside
the nozzle hooked to amicroprocessor.
It's purely mechanical -- and ingenious.

Near
the tip of the nozzle is a small hole, and a small pipe leads back from the hole
into the handle. Suction is applied to this pipe using aventuri.
When the tank is not full, air is being drawn through the hole by the vacuum,
and the air flows easily. Whengasolinein
the tank rises high enough to block the hole, a mechanical linkage in the handle
senses the change in suction and flips the nozzle off.

Here's
a way to think about it -- you've got a small pipe with suction being applied at
one end and air flowing through the pipe easily. If you stick the free end of
the pipe in a glass of water, much more suction is needed, so a vacuum develops
in the middle of the pipe. That vacuum can be used to flip a lever that cuts off
the nozzle.

The
next time you fill up your tank, look for this hole either on the inside or the
outside of the tip.

How
Gasoline Works

In
theUnited
Statesand the rest of the
industrialized world, gasoline is definitely a vital fluid. It is as vital to
the economy asbloodis
to a person. Without gasoline (anddiesel
fuel), the world as we know it would grind to a halt. The U.S. alone
consumes something like 130 billion gallons (almost 500 billion liters) of
gasoline per year!

What
is it in gasoline that makes it so important? In this article, you will learn
exactly what gasoline is and where it comes from.

Is
the United States addicted to gasoline?

Ah,
petroleum -- used in everything from lipstick and lubricants to motor oil and
medications, oil is one product the world just can't seem to get enough of. TheUnited
Statesespecially, which
consumes roughly 21 million barrels of the stuff a day, has quite an attachment
to this ubiquitous product [source:EIA].
And while oil can be refined into a variety of products, Americans seem to
prefer theirs in the form ofgasoline.
In fact, the United States consumes more gasoline thanSouth
America,Europe,AfricaandAsiacombined
[source:EIA].

So
what's with the United States and its gasoholic tendencies? Is the country truly
addicted to gasoline, and if so, what factors led it to get hooked?

While
the United States obviously has quite a fixation with the amber liquid, its
fondness for gasoline probably doesn't fit the official criteria for an
addiction. Rather, the affinity is more like a bad habit spurred on by a number
of government policies put into place over the years. Combine a relatively
wealthy nation with low fuel taxes, low fuel efficiency requirements and a poor
public transportation system, and you have the perfect climate for a gasoline
obsession.

As
opposed to other countries likeDenmark,
where high purchase taxes oncarscan
deter driving, the United States has few roadblocks to impede their gas-guzzling
ways. Quite the opposite, in fact -- with a vast road system crisscrossing the
country and relatively cheap fill-up stations every few miles, what are American
citizens to do? Why, drive of course! And drive they do, as there are more than
244 million vehicles roaming U.S. highways -- 755 cars for every 1,000 people
[source:DOT,Pentland].

Lots
of cars don't automatically equal high gasoline consumption though. ConsiderPortugal,
which has 773 cars for every 1,000 people, yet consumed less than 45,000 barrels
of gasoline a day in 2004 [source:Pentland,≠EIA].
True, the United States is much larger than Portugal, but that's not the only
reason its gasoline consumption far outpaces every other nation. Despite the
fact that Americans now own fewer vehicles than they used to, the vehicles they
do own travel farther and require more gasoline than those of any other
industrialized nation [source:Pentland].

Why
the discrepancies? Keep reading to find out.

What
speed should I drive to get maximum fuel efficiency?

This
is actually a pretty complicated question. What you are asking is what constant
speed will give the best mileage. We won't talk about stops and starts. We'll
assume you are going on a very long highway trip and want to know what speed
will give you the best mileage. We'll start by discussing how much power it
takes to push the car down the road.

Thepowerto
push a car down the road varies with the speed the car is traveling. The power
required follows an equation of the following form:

road
load power = av + bv≤ + cv≥

The
lettervrepresents
the velocity of the car, and the lettersa,bandcrepresent
three different constants:

Theacomponent
comes mostly from the rolling resistance of the tires, and friction in the
car's components, like drag from the brake pads, or friction in the wheel
bearings.

Thebcomponent
also comes from friction in components, and from the rolling resistance in
the tires. But it also comes from the power used by the various pumps in the
car.

Theccomponent
comes mostly from things that affect aerodynamic drag like the frontal area,
drag coefficient and density of the air.

These
constants will be different for every car. But the bottom line is, if you double
your speed, this equation says that you will increase the power required by much
more than double. A hypothetical medium sized SUV that requires 20horsepowerat
50 mph might require 100 horsepower at 100 mph.

You
can also see from the equation that if the velocityvis
0, the power required is also 0. If the velocity is very small then the power
required is also very small. So you might be thinking that you would get the
best mileage at a really slow speed like 1 mph.

But
there is something going on in theenginethat
eliminates this theory. If your car is going 0 mph your engine is still running.
Just to keep the cylinders moving and the various fans, pumps and generators
running consumes a certain amount of fuel. And depending on how many accessories
(such as headlights and air conditioning) you have running, your car will use
even more fuel.

So
even when the car is sitting still it uses quite a lot of fuel. Cars get the
very worst mileage at 0 mph; they use gasoline but don't cover any miles. When
you put the car in drive and start moving at say 1 mph, the car uses only a tiny
bit more fuel, because the road load is very small at 1 mph. At this speed the
car uses about the same amount of fuel, but it went 1 mile in an hour. This
represents a dramatic increase in mileage. Now if the car goes 2 mph, again it
uses only a tiny bit more fuel, but goes twice as far. The mileage almost
doubled!

What's
the difference between gasoline, kerosene, diesel, etc?

The
"crude oil" pumped out≠ of the ground is a black liquid calledpetroleum.
This liquid containsaliphatic
hydrocarbons, or hydrocarbons composed of nothing≠ but hydrogen
and carbon. The carbon atoms link together in chains of different lengths.

It
turns out that hydrocarbon molecules of different lengths have different
properties and behaviors. For example, a chain with just one carbon atom in it
(CH4)
is the lightest chain, known as methane. Methane is a gas so light that it
floats likehelium.
As the chains get longer, they get heavier.

The
first four chains -- CH4(methane),
C2H6(ethane),
C3H8(propane)
and C4H10(butane)
-- are all gases, and they boil at -161, -88, -≠46 and -1 degrees F,
respectively (-107, -67, -43 and -18 degrees C). The chains up through C18H32or
so are all liquids at room temperature, and the chains above C19are
all solids at room temperature.

So
what's the real chemical difference between gasoline, kerosene and diesel? It
has to do with their boiling points. We'll get into that on the next page.≠

What
does octane mean?

If
you've readHow
Car Engines Work, you know that almost all cars use four-strokegasolineengines.
One of the strokes is thecompression
stroke, where the engine compresses a cylinder-full of air and gas into
a much smaller volume before igniting it with aspark
plug. The amount of compression is called thecompression
ratioof the engine. A
typical engine might have a compression ratio of 8-to-1.

Theoctane
ratingof gasoline
tells you how much the fuel can be compressed before it spontaneously ignites.
When gas ignites by compression rather than because of the spark from the spark
plug, it causesknockingin
the engine. Knocking can damage an engine, so it is not something you want to
have happening. Lower-octane gas (like "regular" 87-octane gasoline)
can handle the least amount of compression before igniting.

The
compression ratio of your engine determines the octane rating of the gas you
must use in the car. One way to increase thehorsepowerof
an engine of a given displacement is to increase its compression ratio. So a
"high-performance engine" has a higher compression ratio and requires
higher-octane fuel. The advantage of a high compression ratio is that it gives
your engine a higher horsepower rating for a given engine weight -- that is what
makes the engine "high performance." The disadvantage is that the
gasoline for your engine costs more.